Demands for full-color prints include some occasions where a volume of orders placed is as small as about several tens of sets to hundreds of sets and beautiful color reproduction is requested. For example, in the order placement for leaflets or posters each containing a company logo, high quality is requested including a requirement for high fidelity color reproduction for a logo that reflects a company color, contrary to a small amount of orders placed, which has been a nuisance for a printing shop where a large number of prints are brought out by making a plate.
In the meantime, even in the field of an image forming technology by electrophotographic process, there has come an opportunity to enter into the print industry where there is a demand for the high speed creation of prints having high resolution and broad color reproduction area by a full-color image forming apparatus employing color toner, owing to the progress of digitalization of recent date.
As one of full-color image forming methods, there is an image forming method called a tandem system. This method is one wherein toner images of respective colors are formed respectively on a plurality of photoreceptors, and these color images are superimposed on an intermediate image transfer body or on a recording sheet, to form a full-color image (for example, see Patent Document 1). The tandem system is better suited for high speed printing because monochrome images and color images can be printed at the same speed in the tandem system. Specifically, based on image information corresponding to four colors of yellow, magenta, cyan and black, electrostatic latent images are formed on respective photoreceptors, and these electrostatic latent images are made to be toner images of respective colors by yellow toner, magenta toner, cyan toner and black toner. Then, these toner images are superimposed on an intermediate image transfer body or on a recording sheet to form a color image.
On the other hand, there is a machine model called a high-end machine that is requested to reproduce subtle hue on a high fidelity basis, in digital full-color image forming apparatuses (for example, see Patent Document 2). The equipment of this kind needs color toner capable of reproducing microscopic dot images and subtle hue on a high fidelity basis. However, in the color reproduction by the toner of four colors of yellow, magenta, cyan and black, a color area capable of reproducing is limited, and there has been a problem in preparation of printed matters wherein expression of subtle hue like the aforesaid company logo mark is often required.
For this problem, there is a technology capable of reproducing subtle hue on a high fidelity basis by expanding a color area in the course of color reproduction, which is represented by the existing full-color image forming method wherein hypochromic color toners are used together to interpolate color reproduction by four colors (for example, see Patent Documents 3 and 4).
As stated above, the full-color image forming technology by electrophotographic process has come to a level to reproduce subtle hue on a high fidelity basis by expanding a color area. However, the aforesaid technology is in the occasion where PPC paper (Plain Paper Copy) developed for electrophotography is used, and there has been no suggestion that excellent color reproduction can also be achieved even when a sheet for offset printing is used. Namely, it is inefficient for a person in printing business to conduct printing by changing a sheet for each equipment, and therefore, forming of toner images capable of reproducing colors in a broad color area by using a sheet for offset printing has been demanded.
In the meantime, a sheet for offset printing has two types including a glossy coated paper and a non-coated paper, and there exist the following problems to be solved for forming toner images on these sheets for offset printing on the image forming apparatus of an electrophotographic process.
The first problem is that the glossy coated paper for offset printing cannot withstand heating and pressure fixing in the image forming process of an electrophotographic process. On the glossy coated paper for offset printing, there are coated resin emulsions such as wax having a melting point of 100-160° C. and polyacrylamide, thus, when intense heat is applied, a glossy layer is damaged, resulting in disturbed gloss. In addition, moisture remaining in cellulose pulp fibers representing a base changes into moisture vapor in the course of fixing to jet out, which causes a problem that image defect of white spots called toner blisters is generated. On the other hand, there is available a sheet for exclusive use which is coated so that moisture vapor can be transmitted, for electrophotography. However, this sheet for exclusive use has not been accepted easily by commercial printers, for the reasons that two-sided printing is extremely difficult in terms of specifications, and texture is different from a sheet for offset printing.
The second problem is that an amount of moisture in the non-coated paper for offset printing tends to fluctuate. In offset printing, hydrophilic property of the paper surface is enhanced, because immersion water is used for printing. Further, paper strength agents are added so that fillings may not be exfoliated from fibers on the paper surface, even when the sheet gets wet with water. As paper strength agents, polyvinyl alcohol has come to be used recently for enhancing paper-making speed, although cationic starch has been used in the past. Since an amount of moisture tends to fluctuate under the influence of paper strength agents, and paper resistance is low in the case of the non-coated paper as stated above, if the non-coated paper is used for image forming of electrophotographic process, transferability tends to be fluctuated. Further, since paper strength agents are not designed under the assumption of heating, it is necessary to consider not to cause thermal denaturation.
[Patent Document 1] Japanese Patent O.P.I. Publication No. 10-20598
[Patent Document 2] Japanese Patent O.P.I. Publication No. 2005-157314
[Patent Document 3] Japanese Patent O.P.I. Publication No. 2004-118020
[Patent Document 4] Japanese Patent O.P.I. Publication No. 2004-142153